The Fischer Tropsch reaction converts syngas (a mixture of CO, H2 and CO2) to hydrocarbons over a solid catalyst. The Fischer Tropsch reaction is extremely exothermic. The Fischer Tropsch reaction is unusual in that many products are produced, including alkanes, alkenes and alcohols, and a wide range of carbon numbers of these hydrocarbons. The product distribution is described by the Schultz Flory distribution, or modifications of this, including the 2 alpha models. Depending on the operating temperature, the products will comprise mainly vapour phase products (T>300° C.) or alternatively a mix of vapour and liquids products (T<250° C.).
The chain length of products includes methane (C1) and ranges to oils and waxes (C>100). Thus the product covers a wide range of boiling points. In particular, certain products, such as methane, are mainly in the gas phase while the heavier products (waxes) are mainly in the liquid phase.
The reaction is often performed in a fixed bed reactor. In many situations, the catalyst is placed in the tube side. The flow, temperature profile and heat transfer in the tube is very complicated. In particular there is three phase flow including solid (catalyst), liquids (heavy hydrocarbons) and vapour/gas (light hydrocarbons and syngas).
The temperature profile in the tube limits the reaction rate that can be obtained in the reactor. Furthermore this limits the productivity of the tube and thus the fixed bed reactor. The maximum temperature is often found close to the inlet of the tube, and the operating temperature (often determined by the temperature of the steam in the shell) is set so as to ensure that this maximum temperature is within certain ranges.
If the reactor temperature is increased, this leads firstly to an increase in production of lighter (undesirable) components and in particular methane and CO2, and at even higher temperatures, deactivation of the catalyst.
Further the local reaction rate decreases as the average radial temperature and reactant concentrations decreases. This leads to lower average production rates in the catalyst bed and thus the reactor.
The inventor is aware of fixed bed reactor designs. Existing reactor designs uses an outer shell as main heat extraction mechanism with a plurality of tubes disposed longitudinally inside the reactor to enhance heat exchange between reactants and the fixed bed.
Heat generated in the tubes is conducted to the fixed bed and the heat is then removed from the fixed bed through the outer shell of the reactor.
However, often the heat exchange designs of the reactors are complex due to the fact that the heat is only removed from the shell of the reactor. This leads to hot spots inside the reactor. Various designs have been focusing on the effective conduction of heat in a radial direction towards the outer shell of the reactor, however the effective heat conduction remains a shortcoming in many reactors.
The present invention aims to address the problem of heat concentrations inside the fixed bed reactors.